1. Field of the Invention
The present invention relates to a method and apparatus for recovering quickly from a failure caused by a faulty subscriber equipment in a passive optical network in which a plurality of subscriber equipments are connected to central office equipment via an optical coupler (star coupler) constructed from a passive device.
2. Description of the Related Art
Previously, for a subscriber requiring high-speed, large-capacity communications, it was common to lay an optical fiber point to point from the central office to serve the subscriber. In recent years, a passive optical network has been devised and commercially implemented as a system for economically serving subscribers requiring wideband communications. In the passive optical network system, an optical coupler constructed from a passive device is provided between the central office and the subscribers, and a single optical fiber (or two optical fibers to provide redundancy) is laid between the central office and the optical redirector coupler, while dedicated optical fibers for the individual subscribers are laid from the optical coupler to the respective subscribers.
The optical coupler distributes downstream optical signals from the central office to the respective subscribers, and combines upstream optical signals being sent from the subscribers to the central office. As a method for multiplexing a plurality of subscribers on a single optical transmission line, time division multiple access (TDMA) is used for upstream transmission, and time division multiplexing (TDM) or time compression multiplexing (TCM) is used for downstream signals.
By sharing the optical transmission line and the optical subscriber unit at the central office in this way, the system construction cost can be reduced compared with the method that connects each individual subscriber to the central office point to point. Furthermore, using a passive device for the optical coupler serves to enhance system reliability compared with a system designed to multiplex and demultiplex optical signals using an active device.
However, in a passive optical network, sharing the optical transmission line gives rise to a problem. That is, a failure caused by a faulty subscriber equipment affects communications between the central office and other subscriber equipments. For example, when an optical subscriber equipment (Optical Network Unit (ONU)) has gone faulty and, because of malfunctioning of the unit, has emitted an optical signal in a time slot where some other subscriber optical network unit is supposed to emit an optical signal, interference is caused to the communication of the optical network subscriber using a time slot overlapping that time slot, bringing down the communication service. Further, if light is continuously emitted because of a failure of laser beam control circuitry or the like in a subscriber optical network unit, communications of all the subscribers served by the central office through the same optical coupler will be brought down. In such cases, it is not possible to locate from the central office side the faulty subscriber optical network unit responsible for the communication failure. Personnel must be despatched to the premises where the optical network units are installed, to examine each terminal and optical network unit, and it will take a lot of time and labor to recover subscribers' communications from the failure.